1. Field of the Invention
This invention relates to a variable focus optical device having the focus distance made variable, particularly to a variable focus optical device in which the surface shape of the elastic body is varied by permitting the elastic body to be protruded or recessed at the innerside portion of the opening formed in the driving member.
2. Related Background Art
As a variable focus lens having the focus distance made variable, there has been known in the art the so called liquid lens, in which its shape is varied by liquid pressure as disclosed in Japanese Laid-open Patent Publication No. 36875/1980 or one utilizing a piezoelectric member as disclosed in Japanese Laid-open Patent Publication Nos. 110403/1981 and 85415/1983.
However, in the so called liquid lens of the former, liquid reservoir and pressuring device are required and therefore miniaturization of the device can be effected with difficulty, and also it has the disadvantage that plane deformation by gravitational force or vibration is great. Further, in the latter there was the drawback that the variable amount of the focus distance was small.
On the other hand, as a device which can solve these drawbacks, the present Applicant has proposed a variable focus optical device which can obtain a desired focus distance by deformation of an elastic body having light transmissivity, specifically by deforming the optical surface formed by the elastic body at the opening to any desired shape by permitting the elastic body having light transmissivity to be protruded or recessed from the opening (Japanese Laid-open Patent Publication Nos. 84502/1985 and 11120/1985.).
Further, the present Applicant, concerning Japanese Laid-open Patent Publication Nos. 84502/1985 and 11120/1985 as mentioned above, have proposed Japanese Laid-open Patent Publication No. 114802 in which the optical surface formed by the elastic body is used as the reflective surface; Japanese Laid-open Patent Publication No. 114804 in which the tackiness of the optical surface is ameliorated by increasing the elasticity of the optical surface; Japanese Laid-open Patent Publication No. 114805.1985 in which openings are provided as opposed up and down to each other; Japanese Laid-open Patent Publication No. 120301/1985 in which a desired optical surface is made by providing the optical surface with distributed elasticity; Japanese Laid-open Patent Publication No. 120303/1985 in which compaction is attempted by curing the elastic surface other than the optical surface; Japanese Laid-open Patent Publication No. 151603/1985 in which the position of the optical axis is also varied along with the focus distance by varing the direction of the opening; Japanese Laid-open Patent Publication No. 156003/1985 in which a desired optical surface is obtained by making the bottom plate opposed to the opening a convex shape or a concave shape; etc.
Whereas, in the variable focus optical device by use of elastic bodies with the constitutions presently known, during deformation of the elastic body, due to concentration of stress at the peripheral portion of the opening, deformation tends to become greater at the peripheral portion of the opening rather than at around the center of the opening, whereby a non-spherical shape with strong curvature may be formed at the peripheral portion of the opening, and also during deformation, for example, although it is desired to effect deformation while maintaining the spherical plane shape, the surface shape may be changed from spherical plane to non-spherical plane. Thus, it was still insufficient for obtaining desired optical characteristics.
The present Applicant, in order to prevent the elastic body from being shaped in the surface shape to non-spherical plane has previously proposed Japanese Laid-open Patent Publication No. 80863/1985 (U.S. Ser. No. 832,649, filed on Feb. 25, 1986 now U.S. Pat. No. 4,802,746). More specifically, within a vessel constituted of a bottom plate 2 formed of glass, etc. and a side wall 3 is housed a transparent elastic body 1, and the elastic body 1 is constituted of a first elastic body 1a and a second elastic body 1b laminated in the optical axis h direction. Above the elastic body 1 is provided an opened plate 4 freely movably on the side wall 3 as the driving member for deforming the elastic body 1. An opening 4a is formed in the opened plate 4, and the elastic body 1 within the opening 4a is protruded or recessed by the movement of the opened plate 4. FIG. 4 shows the state where no pressure is applied, and by application of a pressure onto the elastic body 1 through the opened plate 4, a part of the elastic body 1 will be protruded so as to become further greater in curvature of the convex lens than the opening 4a. On the contrary, if a negative pressure is applied onto the elastic body 1 adhered to the opened plate 4, the elastic body 1 will be recessed so as to become smaller in curvature at the opening 4a. This behaviour is the same, even when the surface shape under the initial state where no force is applied may be changed to flat or concave plane. The optical device shown in FIG. 4 is constituted such that, of the elastic body 1 comprising a plurality of elastic bodies, the modulus E.sub.A of the first elastic body 1a on the side of the driving member is made greater than the modulus E.sub.B of the second elastic body 1b adjacent to the first elastic body 1a, and the elastic body has a uniform thickness from the center to the peripheral portion.
Further, the present Applicant has proposed an optical device with the storage modulus and the loss modulus of the elastic body being both in the range of 5.times.10.sup.2 to 1.times.10.sup.8 dyne/cm.sup.2 at a frequency in the range of 0.1 rad/s to 1.times.10.sup.3 rad/s, and with the mechanical loss rate being 1 or less at a frequency in the range of 0.1 rad/s to 1.times.10.sup.3 rad/s (Japanese Patent Application No. 119182/1986, U.S. Ser. No. 53,676, filed on May 26, 1987).
Since E.sub.A is greater than E.sub.B in the optical device having the above elastic body constituted of the first elastic body la and the second elastic body 1b, when the opened plate 4 is moved downward in FIG. 4, the elastic body 1b tends to be deformed greater. This deformation will make the interface between the elastic body 1b and the elastic body la a non-spherical plane shape with a strong curvature at the peripheral portion. As accompanied with such deformation, on the elastic body 1a will act the force to bend the elastic 1a by rising at the center portion of the elastic body 1b and the force to increase the area of the interface between the elastic body 1a and the elastic body 1b. When the elastic body 1a is thin, its primary rigidity is the elongation rigidity of the film. Accordingly, at this time, the elastic body 1a tends to make the surface area as small as possible and be deformed approximately in parabolic plane shape. On the other hand, when the elastic body is relatively thick, it will obstruct abrupt change in curvature by its flexural rigidity.
For this reason, the elastic body 1a in either case tends to be deformed in non-spherical plane shape with weak curvature at the peripheral portion as contrary to the elastic body 1b. Therefore, if the effect of making the elastic body 1a non-spherical plane with strong curvature at the peripheral portion is balanced with the effect of making it non-spherical plane with weak curvature at the peripheral portion, the surface of the elastic body 1a within the opening 4a will be deformed while maintaining a shape approximate to spherical plane.
Whereas, for the above optical device to be utilized widely for photographing optical systems such as camera, video, etc., it would be desirable to make thinner the device itself. However, in the optical device shown in FIG. 4, this can be done with difficulty. More specifically, in the optical device as described above, the elastic body with greater amount of deformation is required to have particularly good elastic characteristic and for that purpose it is desirable to reduce the change rate during deformation. Accordingly, the thickness of the elastic body 1b must be ensured to some extent. For this reason, when it is desired to make thinner the optical device, there is employed the constitution in which the thickness of the elastic body 1a is preponderantly made thinner. For obtaining a shape approximate to spherical plane for the surface of the elastic body 1a while making thinner the elastic body 1a, according to the analysis by use of the finite element method, it is required that the modulus ratio E.sub.A /E.sub.B of the elastic body 1a and the elastic body 1b should be increased, even to 400 to 1000 in an extreme case.
On the other hand, as the material to be used for this optical device, silicone rubber may be preferably employed, and a plurality of elastic bodies with different moduli can be obtained by varying its crosslinking degree. And, the modulus obtained at this time may be about 3.times.10.sup.3 to 2.times.10.sup.6 dyne/cm.sup.2, and a modulus ratio up to about 700 may be obtained as the material itself. However, when the elastic body is
made to have a modulus of about 3.times.10.sup.3 dyne/cm.sup.2, its viscosity will become remarkably stronger to deteriorate response, and also due to the state approximate to liquid, molding and armoring thereof can be done with difficulty. Further, it is susceptible to the influence from gravitational force, whereby deformation by the weight of its own may also occur. Also, when the elastic body 1a is constituted to have a modulus of about 2.times.10.sup.6 dyne/cm.sup.2, toughness is deteriorated as undesirable with respect to durability.
For the reasons mentioned above, it is difficult to realize an optical device with thin film formation by a combination of a modulus ratio E.sub.A /E.sub.B of 150 or higher.